1 1 8 Proceedings of the British Association. 



zeal of the distinguished Herschel and his no less distinguished and 

 accomplished son. The subject of double and multiple stars pro- 

 mised a rich harvest, if our instrumental powers could be enlarged 

 to any considerable extent ; and another field, no less promising, was 

 that of nebulae, of which some of those examined by the Herschels 

 seemed to lay open to the contemplation of the astronomer regions 

 in comparison with which our entire sidereal sphere might be consi- 

 dered as a mathematical point. Now, in examining these, he did not 

 mean to deny that accurate measurements were of much importance 

 — indeed, of the very highest ; but it must be obvious, that before we 

 can measure we must be rendered capable of seeing. Here, then, he 

 found the strongest inducement to attempt to improve the instru- 

 ment by which this was to be accomplished. Two objects required 

 to be kept in view : first, to give the telescope sufficient aperture to 

 secure a sufficiency of light ; secondly, to increase to a sufficient ex- 

 tent the magnifying power. On these depended what might be 

 called the optical power of the instrument, but particularly upon the 

 former. For instance, the large telescope, of which a model stood 

 before them, to be used effectually, must have a magnifying power of 

 300 times. Now, another instrument, very inferior in size, might 

 have a much higher power, but, from the vast quantity of light 

 which it collected into the image, objects in it became distinct which 

 could not be at all seen by those of inferior aperture. The next 

 question he had to determine was, whether he should attempt re- 

 fractors or reflectors. Just at that time very large and very fine 

 discs of the proper glass had been produced upon the Continent, and 

 a strong hope was entertained of bringing the refracting telescope to 

 a degree of perfection which had been hitherto rather hoped for than 

 attained. But, upon a calm balancing of all the difficulties which 

 opposed their construction, he determined to attempt the improvement 

 of the Newtonian reflector, and that notwithstanding it was well 

 known that an error of form of the reflector produced an error in the 

 image more than five times as great as the same error in the refractor 

 would produce. It was to the steps by which he attained this object 

 that he was now about to direct the attention of the section. 



" Having concluded that upon the whole there was a better pros- 

 pect of obtaining by reflection, rather than by refraction, the power 



